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SALTS AND TRACE ELEMENTS 361 10 Salts and Trace Elements The salt content (salinity) of soils and the frequently associated drainage problems are pervasive in agriculture. Salinity affects germination as well as seedling and vegetative growth. It also reduces crop yields. Waterlogging causes poor aeration in the root zone, with effects ranging from reduced growth to death of plants. Some constituents of salinity, such as sodium, may have a deleterious impact on the physical condition of the soil, impairing crop production. About 19 million ha (47 million acres), or 10 percent of U.S. cropland, is irrigated (Postel, 1990). About 5.2 million ha (14 million acres) of this irrigated land is currently affected by salt. Salinity problems are not restricted to irrigated areas and arid climates. Halvorson (1990) estimates that nearly 1 million ha (2.5 million acres) of productive agricultural dryland in the western United States has been salinized through saline seeps. Salinity problems also occur in humid regions through, for example, seawater intrusion into low-lying coastal farmlands. Soil salinization is a worldwide problem. According to Szabolcs (1989), about 1 billion ha (25 billion acres) of the world's soils are affected by salt. Saline and sodic (sodium-containing) soils cover about 10 percent of the world's arable lands and exist in 100 countries. Szabolcs (1989) reports that, worldwide, some 10 million ha (25 million acres) of irrigated land is abandoned annually because of salinization, sodification, and waterlogging. Worldwide, in the arid-to-semiarid regions where irrigation is practiced, soil salinization and the frequently accompanying waterlogging
SALTS AND TRACE ELEMENTS 362 problem have plagued agriculture for centuries. Historical records from the past 6,000 years reveal that many ancient civilizations whose existence was based on irrigated agriculture have failed, for example, the Sumerian civilization in the Mesopotamian Plains in Iraq, the Harappa civilization in the Indus Plain region in India and Pakistan, the inhabitants of the lower Viru Valley in Peru, and the Hohokam Indians in the Salt River region in Arizona (Tanji, 1990). Salinization and waterlogging are not unique to ancient civilizations, however. For example, a critical agricultural and ecological crisis exists in the San Joaquin Valley of California (San Joaquin Valley Drainage Program, 1990). Of the nearly 1 million ha (2.5 million acres) of irrigated cropland on the valley's west side, about 38 percent is waterlogged and 59 percent has increased levels of salt. About 54,000 ha (133,000 acres) of the drainage-impacted lands have tile drainage systems, but only about 21 percent of the entire west side, however, can discharge its saline subsurface drainage into the San Joaquin River for eventual disposal into the Pacific Ocean. Adding to the difficulties in managing irrigation-induced water quality problems was the discovery of selenium and other toxic trace elements in subsurface drainage waters in the San Joaquin Valley's west side (National Research Council, 1989b). Since the 1983 discovery of selenium poisoning of waterfowl at the Kesterson National Wildlife Refuge in the San Joaquin Valley, the U.S. Department of the Interior has implemented the National Irrigation Water Quality Project. Twenty-six sites in 15 western states are being investigated to ascertain whether selenium and other trace elements in irrigation drainage water are causing ecological damage (Engberg et al., 1991). The National Irrigation Water Quality Project has found selenium in detectable quantities in 20 reconnaissance study sites. Of the principal trace elements, selenium appears to pose the greatest potential toxic effects on aquatic biota. The primary source of selenium in the drainage waters of the San Joaquin Valley is the Cretaceous-period marine sedimentary shales of the Coast Range mountains. The ecological hazards of potentially toxic trace elementsâsuch as selenium, boron, arsenic, molybdenum, mercury, vanadium, and uraniumâare magnified when agricultural drainage waters are disposed into hydrologically closed basins and sinks and accumulate in the food chain. This chapter describes the sources of salts and trace elements and their effects on soils and plants. This chapter also explores alternative management options that can be used to minimize the irrigation-induced water quality problems and sustain irrigated agriculture. Because of the complexities of the salinity, drainage, and toxic element problems, the chapter begins with an overview.
SALTS AND TRACE ELEMENTS 363 Control of salinity is a major problem facing irrigated agriculture in the western United States. A USDA technician in California examines land on which productivity has been seriously damaged by salinity. Credit: U.S. Department of Agriculture. OVERVIEW OF SALINITY AND DRAINAGE PROBLEMS Producers in arid and semiarid regions who irrigate their agricultural lands must always deal with salinity. This well-established fact is often overlooked. The Salinization Process The salinization process begins with snowmelt that runs off mountains in rills, forms streamlets, and then rushes down with ever greater
SALTS AND TRACE ELEMENTS 364 force. It dissolves small amounts of minerals from the riverbed and soils with which it comes into contact, slowly cutting out a channel along its path. Some of the water penetrates the soil or the underlying formations, reemerging as stream flow farther downstream or recharging the groundwater basin; again, some of the minerals that come into contact with the water on the way are dissolved and carried along. Elsewhere, as in the extensive Mancos shale region of Utah and Colorado, percolating waters displace groundwater that, because of its marine origin, contains high levels of salts. Whatever its origin, water tapped for irrigation contains salt. Sometimes, the amounts are very small, but in other cases they are substantial. When this water is applied to the land to nourish crops, much of it is taken up by plants and is returned to the atmosphere. Since only pure water evaporates from the soil surface or transpires from plant surfaces (evapotranspiration), the evapotranspiration process leaves the salts behind in the soil. Thus, irrigation automatically and relentlessly leads to an accumulation of salts in the soil. Unless provision is made to leach a portion of this accumulating salt out by means of rainfall or the application of irrigation water in excess of crop water needs, the soil will soon become too saline for crop growth. Such excess water will eventually lead to a rising water table and waterlogging unless there is adequate drainage. Thus, all irrigators must cope with salinity and all irrigated fields need drainage, be it natural or provided by producers. The drainage water tends to be substantially more saline than the original irrigation water. Suppose a producer irrigates his or her land with water containing 250 mg/L of total dissolved solids per liter (250 ppm) and suppose that three-fourths of this water is evapotranspired and one-fourth remains for leaching, then, as a first approximation, the drainage water will contain 1,000 mg/L of total dissolved solids per liter (1,000 ppm). Drainage This simple sketch of the salinization process provides a valid overall picture, but it must be modified in numerous ways to be accurate and useful for devising and understanding practical management schemes. It suffices, however, to make clear that irrigation in arid regions is short-lived unless drainage is provided. As a consequence, irrigation always leads to the need to dispose of drainage water and thus brings about potential off-site water quality degradation. In a typical situation, drainage water is discharged downstream from an irrigation site into a stream that will again be used for
SALTS AND TRACE ELEMENTS 365 irrigation, and this cycle can be repeated a number of times. Each time, more salt is added to less water. Sources of Salinity The occurrence of saline soils is not restricted to irrigated areas. The same process of mineral weathering or dissolution and subsequent concentration because of water evaporation often leads to high salt levels in soils of arid and semiarid regions. The scarcity of rain that makes these areas arid restricts the possibility of leaching and thus leads to salt accumulation. Additional secondary sources of salts include atmospheric deposition and seawater intrusion. An example of increased salinity from atmospheric deposition is found in western Australia, where chloride-dominated salinity has been attributed to substantial deposition by wind over time of salts predominantly made up of sodium chloride derived from the Indian Ocean. A special case of dryland salinity of particular concern in both North America (Halvorson, 1990) and Australia (Sharma and Williamson, 1984) is that of saline seeps. A saline seep occurs when water in excess of that required by plants percolates below the root zone and, upon encountering some type of barrier or restricting layer, moves laterally downhill and emerges in a seepage area, having picked up dissolved solids in transit. The nature of the restricting layer can vary from a change in soil texture to a coal seam or a change in geologic structure. Saline seeps are often encountered where farmers practice a wheat-fallow rotation; during dry periods, such a rotation may serve to conserve some water during the noncropped period to aid the following crop, but in somewhat wetter years, the precipitation in excess of that required by plants initiates the process that leads to a seep. In the U.S. northern Great Plains, the extent of drainage from seeps has been estimated to have affected an area of about 1 million ha (2.5 million acres) (Miller et al., 1981). The problem can sometimes be corrected by installing an interceptor drain (Doering and Sandoval, 1976). The disposal of the drainage water, however, creates a new problem; and the cost of providing an outlet is often excessive. A less costly solution is found by switching to a flexible cropping system in the recharge area (the area of land that is the source of water for the seep) to ensure that the crops grown in sequence use all of the available soil water. The systems must be flexible to adapt to the vagaries of the weather: different cropping sequences are required if precipitation has been above- average than are required if precipitation has been below-average (Black et al., 1981).
SALTS AND TRACE ELEMENTS 366 Effects of Salinity The effects of typical salts on agricultural crops and soils have long been known, and the need for drainage has been advocated for well over a century. Explicit concern about the off-site effects of salinated drainage waters, however, is more recent. Passage of the Colorado River Salinity Control Act (PL 93-320) in 1974 illustrates a formal awakening to the cumulative downstream problems of the stepwise consumption of a river's water. This law prescribed a process for reducing the salinity in the Colorado River to protect downstream users. In studies conducted for that purpose, investigators found that about 37 percent of the salt in the lower part of the river could be attributed to irrigation; they also ascertained that most of the adverse effects from higher salt levels were suffered by those using the water for municipal or industrial rather than for agriculture purposes (Jones, 1984). Irrigation drainage also may contain nitrates and various pesticides (see Chapters 6 and 8, respectively). The present discussion, however, is restricted to salts and trace elements that are unique to irrigated agriculture. For well over a century, agricultural producers have been concerned with the adverse effects of salinity on their irrigated agricultural lands, and they have developed detailed management schemes to deal with these problems. For at least 25 years, investigators have given serious attention to the off-site problems (downstream water degradation) associated with irrigation. More recently, however, a new concern has been added: the presence of toxic trace elements in soils and shallow groundwaters. Toxic Trace Elements The term salinity has been loosely defined in terms of the major anions and cations found in irrigation water, with careful attention given to some specific ion effects such as those from sodium and chloride. The toxic effects (to plants) of low levels of boron have also been of concern in certain areas; but investigators assumed that, other than boron, trace elements did not occur in sufficient concentrations to be of concern. This changed in 1982 when investigators discovered that the elevated concentration of selenium in the Kesterson Reservoir was causing reproductive failures in aquatic organisms and waterfowl. This reservoir was in fact a set of shallow ponds used to store and evaporate agricultural drainage water (National Research Council, 1989b).
SALTS AND TRACE ELEMENTS 367 Extensive salinity damage is apparent in this cropland in California's Coachella Valley. Barren areas within the crop rows show where soil is too damaged to sustain plant life. Credit: Agricultural Research Service, USDA.
SALTS AND TRACE ELEMENTS 368 Sources of Trace Elements Since 1982, selenium as well as several other trace metals have been found with some regularity in drainage waters in certain geographic areas (Deason, 1989). Trace elements are natural constituents of the soils or the underlying geologic materials and may be mobilized by irrigation, in contrast to the major salts discussed above. Trace elements also differ from the major salts in terms of their typical concentrations. Whereas an investigator may measure salinity in 100s or 1,000s of milligrams per liter (ppm), a typical selenium concentration may be between 10 and 100 µg/liter (10 and 100 ppb). Furthermore, the effect of excessive salinity is likely to be loss of crop yield or damage to plumbing, whereas high levels of selenium or molybdenum can be toxic to fish and waterfowl, causing substantial harm to the ecosystem and, possibly, humans. Reducing the Impacts of Trace Elements Irrigation practices can be altered in numerous ways to minimize the adverse effects of salinity on agricultural lands, and similarly, management can reduce water quality problems (Suarez and Rhoades, 1977). The same can be said for trace elements, although the specific management or corrective practices may differ. It is not possible, however, to completely eliminate these effects or to make them cost-free. It is an important principle that irrigation of arid lands always degrades water quality: ''irrigation agriculture over time cannot avoid causing an adverse off-site effect. This effect must be acknowledged: it can be minimized, internalized, or rejected, but it cannot be ignored. If irrigation is a desired use of water, then its waste waters must be treated and/or disposal provided for" (National Research Council, 1989b:41). It seems appropriate to elaborate on this tenet with some examples. The discussion above suggested that irrigation in the upper Colorado River has led to substantial damage to plumbing in Los Angeles. It is possible, for example, to substantially increase the irrigation efficiency in the Colorado River's Grand Valley and to line the delivery canals to reduce the amount of seepage water from ditches, canals, and farm fields that returns to the river. Such actionsâ now under wayâwill be effective in reducing the salt contribution to the river from the Grand Valley, estimated at 530 million kg/year (240 million lb/year), because a reduction in seepage flow should lead to a proportional reduction in salt discharge masses (Inman et al., 1984). However, it is not possible to eliminate all salt contributions from the Grand Valley to the Colorado River.
SALTS AND TRACE ELEMENTS 369 Another situation is encountered in the Imperial Valley of California (Meyer and van Schilfgaarde, 1984). There, the drainage water from the irrigated valley enters the inland Salton Sea. This sea has no outlet; the only water loss is by evaporation. Excessive irrigation (and drainage) raises the level of the sea, damaging the lands adjacent to the sea. Increases in irrigation efficiency lower the sea level but accelerate its rate of salinization. Either way, the salt concentration continues to increase, reducing the Salton Sea's value as a habitat for fish and wildlife. Were it not for irrigation, the sea would be dry. Thus, there is a dilemma. Irrigation originally formed the sea in 1902, but continued irrigation will reduce the sea's biological value. It is unlikely that irrigation is sustainable without environmental insult. Irrigation in the Imperial Valley would be sustainable (that is, it could be continued indefinitely) if society were willing to sacrifice the sea, which would not exist if it were not for irrigation. Irrigation causes off-site damages in terms of decreased water quality. Society must weigh the benefits accrued from irrigation against the disadvantages associated with and the costs of reducing soil salinization and water pollution (van Schilfgaarde, 1990). SOURCES AND EFFECTS OF SALINITY Nature of Salinity The salinity in soils and waters is made up of dissolved mineral salts. The major cations are sodium, calcium, magnesium, and potassium; the major anions are chloride, sulfate, bicarbonate, carbonate, and nitrate. The concentrations of these solutes are reported in milligrams per liter, millimoles per liter, or milliequivalents per liter. Salinity is typically expressed as a lumped salinity parameter: electrical conductivity (EC) or total dissolved solids (TDS). EC is an intensive electroconductivity measure expressed in microSiemens per centimeter (µS/cm) for soils and waters with lower salinity levels and deciSiemens per meter (dS/m) for soils and waters with higher salinity levels. TDS is an extensive gravimetric measure reported in milligrams per liter, or grams per liter for hypersaline waters. Although no exact relationship exists between EC and TDS, the number of milligrams of TDS per liter can be approximated by multiplying EC (in dS/m) by a factor of 640 for many waters to a factor of 800 for hypersaline waters. Measuring EC and TDS Measuring EC and TDS in waters is straightforward, but it is not in soils because salinity is significantly affected by the prevailing soil
SALTS AND TRACE ELEMENTS 370 moisture content. The concentrations of salts in the soil solution do not typically change in direct proportion to changes in soil water content because the major solute species participate in such mechanisms as mineral precipitation and dissolution, cation-exchange, and ion association. Moreover, soil salinity is a dynamic property since soluble salts are highly mobile in the soil profile. Soil salinity is typically measured in the laboratory by obtaining an extract from a soil sample that is moistened with distilled water to a reference saturated soil water content. The EC of the soil saturation extract is reported as ECe (electrical conductivity of saturation extract). EC in field soils may be obtained by vacuum extraction of the soil solutions from recently wetted soils and is reported as ECss (electrical conductivity of the soil solution). In situ EC measurements of moist field soils can be also obtained by using the four- electrode salinity probe and the electromagnetic device, which give the bulk EC of soils reported as ECa. The time domain reflectrometry is a promising method of measuring salinity and water content independently with the same probe (Dalton et al., 1984). Sources of Salinity A primary source of salts is chemical weathering of the minerals present in soils and rocks. The more important chemical weathering mechanisms include dissolution (contact with water), hydrolysis (reaction with water), carbonation (reaction with dissolved carbon dioxide), acidification (reaction with proton), and oxidation-reduction (transfer of electrons). All of these reactions contribute to an increase in the dissolved mineral load in the soil solution and in waters. Other important sources of salts include fossil salts (for example, salt domes), secondary deposits from marine or lacustrine (lake) environments (for example, gypsum), wet and dry atmospheric deposition, and salts contained in waters applied to the land, rising groundwater levels, soil and water amendments, animal manures and wastes, chemical fertilizers, sewage effluents and sludges, and oil and gas field brines. Effects of Salts on Soils The physical properties of soils that are conducive to potential high-level crop production include adequate permeability of the soil for water and air and the presence of friable (easily crumbled or pulverized) soil for seed germination and root growth. These two properties are the
SALTS AND TRACE ELEMENTS 371 ones most affected on irrigated lands and are reflected as poor permeability and poor soil tilth (tilth is the state of aggregation of a soil). The major factors affecting these physical conditions of soil are electrolyte content and the sodium content of the applied water or soil. The former is evaluated by EC. The latter is evaluated by the sodium adsorption ratio (SAR), which is defined as (ion concentrations are expressed in millimoles per liter); Reduced Water Infiltration A combination of a low EC and a high SAR results in poor water infiltration rates in many soils (Oster and Rhoades, 1984). The deleterious impact of high-SAR water may be partially overcome by increasing the EC of the applied water. The unfavorable chemistry leading to water infiltration problems may be reflected by slaking (breakdown) of soil aggregates and the dispersion and swelling of clay minerals. Slaking and dispersion lead to reduced permeability and poor soil tilth, which, in turn, result in poor crop establishment, inadequate water intake rates, and increased runoff and erosion. Swelling reduces the sizes of the interaggregate pore spaces in the soil and, therefore, produces a substantial, although reversible, reduction in the hydraulic conductivities of soils. Swelling is particularly important in soils that contain expandable clay minerals and have SARs of greater than about 15. Dispersionâthe release of individual clay platelets from aggregatesâand slakingâthe breakdown of aggregates into subaggregate entitiesâcan destroy pore interstices, reducing in an irreversible way the hydraulic conductivity of a soil. Dispersion and slaking can occur at very low SARs if the EC of the soil solution is also very low. Water penetration problems can develop when a seal with very low hydraulic conductivity forms at the soil surface. Seals are generally considered to be wet, and they reduce infiltration and increase runoff and erosion. Crusts are dry seals and, in addition to slowing water penetration, reduce the ability of seedlings to emerge. Structural crusts are preferentially formed in soils exposed to the beating action of falling water droplets (for example, from rain or sprinkler irrigation systems). Surface sealing is enhanced by the dispersion and slaking mechanisms that take place when low-EC, high-SAR waters equilibrate with the soil surface.
SALTS AND TRACE ELEMENTS 372 Depositional crusts form when suspended soil sediments that originated from the detachment of soil materials are deposited at the soil surface as the water infiltrates the soil. Depositional crusts, like structural crusts, are more likely to form when irrigation waters with low ECs are applied to the soil because the dispersion-flocculation status of the suspended sediments determines the hydraulic properties of the depositional crusts that are formed (Shainberg and Singer, 1990). Effects of Salinity and Sodicity Because the effects of salinity and sodicity on soils are interrelated, both the EC and the SAR of the applied water must be considered simultaneously when assessing the potential effects of water quality on soil water penetration. A general and more definite EC-SAR relationship for all soils cannot be developed because of variations in clay mineralogy; clay, organic matter, and sesquioxide contents; and pH. Figure 10-1 shows that salt accumulation patterns in surface soils vary with the method of water application (Ayers and Westcot, 1985). Doneen and colleagues (1960) measured the distribution of salts in the crop root zone in a lysimeter irrigated with about 2 surface meters of irrigation water having an EC of 2.9 dS/m. (A lysimeter is a device that encases a soil profile to provide accurate measurements of water applied to and draining through the soil, and of chemical changes in the soil.) The ECe of the soil profile was initially between 0.4 and 0.6 dS/m. The distribution of salts in this freely drained, cropped soil profile after application of 2 m of irrigation water reveals that with each irrigation there is leaching of salt from the surface soil and subsequent salt accumulation deeper in the soil. The extent of salt accumulation in the lower portion of the crop root zone is dependent on the leaching fraction, which is defined as the ratio of the depth of drainage water past the root zone to the depth of infiltrated water. For soils cropped under shallow water table conditions, there is a tendency for salts to accumulate nearer to the soil surface with a rise in the water table because of the upward evaporative flux rather than downward leaching flux in deeply drained soils (Namken et al., 1969). The extent of soluble salt accumulation and its distribution in the crop root zone will have an impact on crop growth and yield. Effects of Salts on Plants The adverse effects of salts on plants can be divided into three main categories (Figure 10-2): effects on water relationships, effects of specific
SALTS AND TRACE ELEMENTS 373 FIGURE 10-1 Typical salt accumulation patterns in surface soils for various methods of water application. Salinity ranges from low (unshaded) to high (darkened). Arrows indicate the direction of soil water flow. Source: R. S. Ayers and D. W. Westcot. 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29, Rev. 1. Rome: Food and Agriculture Organization of the United Nations.
SALTS AND TRACE ELEMENTS FIGURE 10-2 Detrimental effects of salinity on plant growth. Source: D. Pasternak. 1987. Salt tolerance and crop productionâ A comprehensive approach. Annual Review of Phytopathology 25:271â291. Reproduced, with permission, from the Annual Review of Phytopathology, Vol. 25, © 1987 by Annual Reviews Inc. 374
SALTS AND TRACE ELEMENTS 375 ions, and effects on energy balance (Pasternak, 1987). The relative importance of each of these effects on the overall plant response to salinity may differ sharply from one plant species to another or under different sets of environmental conditions. Effects of Water Relationships Salt in the root zone decreases the osmotic potential of the soil solution and therefore reduces the availability of water to plants. If the osmotic potential of the soil becomes lower than that of the plant's cell, the latter would suffer osmotic desiccation and loss of turgor pressure (turgor pressure is the pressure within a plant cell). To survive, the plant must adjust osmotically to compensate for the lower external water potential. This can be effected by absorption of ions from the medium, synthesis of organic compounds, or both. The synthesis and transport of organic compounds, the transport of inorganic ions, and the biochemical adjustments necessary for plant survival require the expenditure of metabolic energy. This expenditure results in depletion of the energy pools needed for growth (Figure 10-2). Specific effects of ions on plants include direct toxicity because of excessive accumulation of ions in the tissues that may affect various physiological processes and a nutritional imbalance caused by an excess of some particular ions. Effects of Ions The potentially toxic effects of certain ions such as boron, chloride, and sodium are associated with their uptake by roots and accumulation in the leaves. Some herbaceous crops and many woody species are susceptible to the toxicities caused by these ions. Some ions, like chloride, can also be absorbed directly into the leaves when moistened during sprinkler irrigation. In addition, many trace elements (see below) are toxic to plants at very low concentrations. Effects on Energy Balance Salinity disrupts the acquisition of mineral nutrients by plants in two ways (Grattan and Grieve, 1992). First, the ionic strength of the substrate can have direct effects on nutrient uptake and translocation, and second, the interactions of major ions in the substrate (that is, sodium and chloride) can have an effect on nutrient ion acquisition and
SALTS AND TRACE ELEMENTS 376 translocation within the plant. Examples of these effects are sodium ion-induced calcium ion or potassium ion deficiencies and calcium ion-induced magnesium ion deficiencies. FIGURE 10-3 Relative salt tolerances of agricultural crops. Source: Adapted from R. S. Ayers and D. W. Westcot. 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29, Rev. 1. Rome: Food and Agriculture Organization of the United Nations. Effects of Salinity on Crop Yields Crop plants respond to salinity in widely ranging manners because of differences in their abilities to adjust osmotically, enabling them to extract water from saline soil solutions (Figure 10-3). Typical examples of salt- sensitive crops are bean, onion, almond, peach, orange, and grapefruit; moderately sensitive crops include corn, alfalfa, clover, cabbage, lettuce, potato, and grape; moderately tolerant crops include safflower, soybean, wheat, barley, tall fescue, squash, and olive; and tolerant crops include cotton, sugar beet, Bermuda grass, asparagus, date palm, and guayule.
SALTS AND TRACE ELEMENTS 377 The yield response function can be defined as Y = 100 - b(ECe - a), where Y is the relative crop yield, b is the slope of the declining yield with increasing ECe, and a is the salinity threshold value above which crop yield declines. Such yield response functions provide producers with general guidelines on expected yields for specific crops or allow them to pursue leaching of salts to ensure economic crop yields. Figure 10-3 shows that the salinity threshold value varies widely among crop plants. Moreover, the sensitivity of plants to soil salinity may change from one stage of growth to the next. For instance, most plants are fairly salt-tolerant during germination but become quite sensitive to salts during emergence and early seedling growth. Other crops, such as cereals, become quite sensitive to salts in the early reproductive stage, and seed yield may be reduced if soil salinity is at elevated levels. Most crops, however, tend to have increasing salt tolerance with increasing age. SOURCES AND EFFECTS OF TRACE ELEMENTS Nature of Trace Elements Trace elements occur in minute concentrations in the environment, for example, less than 100 mg/kg (100 ppm) in soils and rocks, less than 10 mg/kg (10 ppm) in plant and animal tissues, or less than 1 mg/liter (1 ppm) in water. Elevated concentrations of inorganic trace elements in soils, waters, plants, and animals have been of concern for about a century, but they have been of greater concern in the past several decades. For instance, the discovery of human mercury poisoning in Japan in the 1950s and 1960s (Minamata and Itai-Itai diseases) triggered research into the environmental hazards of mercury in aquatic systems (Adriano, 1986). More recently, the discovery in the early 1980s of selenium poisoning of fish and waterfowl in the Kesterson National Wildlife Refuge has further heightened awareness of the potential hazards of naturally occurring trace elements in agricultural drainage waters (National Research Council, 1989b). Deverel and Fujii (1990) state that the presence of elevated concentrations of trace elements in groundwaters and irrigated soils in the San Joaquin Valley's west side poses threats to agriculture as well as human and animal health. They found that the threat is reflected in three ways. First, trace elements can accumulate in plants to levels that cause phytotoxicity. Second, trace elements in plants can adversely affect humans and animals that consume those plants. Third, trace elements
SALTS AND TRACE ELEMENTS 378 can migrate with seepage through the root zone into groundwaters, possibly reemerging with subsurface drainage in surface waters, thereby affecting wildlife, or with groundwater pumped for domestic use, thereby threatening human health. The U.S. Environmental Protection Agency (EPA) (1986b) considers the following trace elements to be potentially harmful to human health if they appear in drinking water: arsenic, barium, cadmium, chromium, fluoride, lead, mercury, selenium, silver, copper, iron, manganese, and zinc. In addition to the trace elements listed above, other trace elements, including boron, nickel, uranium, tellurium, beryllium, and aluminum, have potential detrimental impacts on aquatic biota (San Joaquin Valley Drainage Program, 1990). Sources of Trace Elements The primary sources of trace elements are earth materials and volcanic emanations. Trace elements are found in primary minerals in rocks and soils frequently as isomorphic substitution (isomorphic substitution is the substitution of one element for another in clay minerals that may result in a net change in the electrical charge of the mineral), for example, lead for potassium in feldspars, selenium for sulfur in pyrite, zinc for magnesium in olivines, and boron for silicon in micas (Sposito, 1989). Trace elements may also coprecipitate with secondary soil minerals, for example, molybdenum and arsenic with iron and aluminum oxides, cobalt and nickel with manganese oxides, vanadium and cadmium with calcium carbonates, titanium with vermiculites, and vanadium and copper with smectites. Numerous trace elements are also associated with soil organic matter, for example, aluminum, vanadium, chromium, manganese, iron, nickel, copper, zinc, cadmium, and lead (Sposito, 1989). These soil minerals serve as reservoirs for the trace elements, which are typically released at a slow rate into soil solutions and waters through a number of chemical weathering mechanisms. Anthropogenic sources of trace elements (Adriano, 1986) include phosphatic fertilizers (zinc, copper, cadmium, chromium, nickel, and lead), liming materials (zinc, manganese, and copper), pesticides (mercury, arsenic, and lead), sewage sludges (cadmium, zinc, copper, lead, and nickel), animal wastes (copper, cobalt, and zinc), coal combustion residues (arsenic, cadmium, molybdenum, selenium, and zinc), mining and smelting residues (lead, copper, zinc, cadmium, cobalt, and manganese), and motor vehicle emissions (lead, zinc, cadmium, and nickel).
SALTS AND TRACE ELEMENTS 379 FIGURE 10-4 Possible abiotic and biotic processes affecting the reactivities and mobilities of trace elements. ECe. electrical conductivity of the saturation extract (dS/m); ECw , electrical conductivity of the irrigation water (dS/m). ECe = 1.5 ECw. Source: K. K. Tanji and L. Vallopi. 1989. Ground water contamination by trace elements. Agriculture, Ecosystems and the Environment 26:229â274. Reprinted with permission from © Elsevier Science Publishers, B.V. Reactivities and Mobilities of Trace Elements The presence or accumulation of trace elements in agricultural drainage waters and groundwaters is influenced by a number of factors, including the nature and sources of trace elements, the particular trace element and its reactivity, and mobility and transport processes. The first item was addressed above. Figure 10-4 represents some of the possible abiotic and biotic processes affecting the reactivities and mobilities of trace elements in soil water systems (Tanji and Valoppi, 1989). The mobile forms of trace elements include various solutes and gaseous species that are subjected
SALTS AND TRACE ELEMENTS 380 to numerous competing reactions. The rate and extent of these reactions are, in turn, influenced by a host of environmental conditions. Because of site-specific conditions and factors and the complexities of the reactivities of trace elements, only a few generalizations are possible. For instance, the cationic trace elements (for example, heavy metals) such as copper, lead, zinc, cadmium, and nickel tend to be strongly retained by soils owing to ion exchange, sorption, and mineral precipitation. In contrast, anionic trace elements (for example, oxyanions) such as selenate (SeO42-), chromate (CrO42-), arsenate (AsO42-), molybdate (MoO42-), and vanadite (VO32-) are subject to greater mobility, even though they may be retained to some extent by clays and sesquioxide surfaces. Differences in mobility, however, exist among the oxyanions of a given trace element. For example, selenite (SeO32-) is more strongly sorbed by soils than selenate (SeO42-), especially in the presence of elevated concentrations of SO42-. Trace elements that tend to form complexes with inorganic and organic ligands have greater mobilities than do those that are not complexed. (Ligands are a group, ion, or molecule coordinated to an element forming a complex.) The solubilities of minerals containing cationic trace elements typically increase as the pH decreases, whereas the mobilities of those containing anionic trace elements typically decrease as the pH decreases. The half-lives of reactions (Langmuir and Mahony, 1985) range from seconds (hydration, acid- base complexation, adsorption and desorption), minutes to hours (oxidation- reduction, gas solution, exsolution), weeks (precipitation, dissolution), months (polymerization and hydrolysis, isotopic exchange), and years (mineral crystallization). Effects of Trace Elements on Soils and Groundwaters To provide examples of the effects of trace elements on soils and groundwaters, this section focuses on selenium, a naturally occurring trace element, and heavy metal accumulation in soils from sludge applications. The distribution of selenium in the San Joaquin Valley in California is shown in Figure 10-5. Figure 10-5A shows the distribution of selenium in the top 30.5 cm (12 inches) of soils in the entire valley floor, Soils in the east side of the valley are extremely low in selenium except in localized sites. Animals that graze on soils on the east side of the valley frequently suffer from selenium deficiency since the selenium-poor soils that formed from sediments from the Sierra Nevada mountain range are mainly granitic. In contrast, soils that formed from sediments derived from the Coast Range mountains are rich in selenium.
SALTS AND TRACE ELEMENTS 381 Figure 10-5B shows the distribution of selenium found in shallow groundwaters on the west side of the San Joaquin Valley. The highest concentrations were in excess of 200 µg/liter (200 ppb) and the maximum was 3,800 µg/liter (3,800 ppb). Drainage water disposed into Kesterson Reservoir had an average selenium concentration of about 300 µg/liter (300 ppb). The soluble selenium fractions in the surface soil depths have been leached in profiles of soils that have been irrigated for decades. In contrast, investigators found elevated selenium levels in surface soil depths when the soils had not been irrigated. The annual precipitation of 100 to 300 mm (4 to 12 inches) is insufficient to leach selenium from surface soils. Based on the ratio of the isotopes, oxygen-18, and deuterium (hydrogen-2) in groundwaters, scientists from the U.S. Geological Survey (for example, Gilliom et al., 1989) have concluded that selenium and other dissolved mineral salts have been concentrated in the shallow groundwaters by evaporation. Chang and colleagues (1984) studied the annual application of a composted sewage sludge and anaerobically digested liquid sludge for 6 years in cropped soils (Figure 10-6). They found that cadmium, nickel, chromium, lead, and copper accumulate almost entirely in the surface 15 cm of the soil. Findings similar to those presented in Figure 10-5 were obtained with the application of liquid sludge. These metals are comparatively immobile because of the strong soil retention mechanisms described above. Effects of Trace Elements on Plants Plants differ in their abilities to absorb, accumulate, and tolerate trace elements. For a given species, concentrations of trace elements in various parts of the plant and among different plant cultivars also vary. Genetically controlled features of plants, morphological and anatomical differences between plants, and the physiology of a plant's ion transport mechanism may be responsible for these differences. Plants can accumulate enough of certain trace elements such as cadmium, selenium, and molybdenum to cause acute toxicity or chronic metabolic imbalances in consumers of the plants. In some cases, plants do not absorb trace elements because they have a soil-plant barrier (Page et al., 1990); in those cases, the food chain is protected from accumulating harmful amounts of trace elements. Ingestion of contaminated soil or dust particles, however, may cause intake of toxic trace elements (Page et al., 1990). The concentration of a trace element that results in toxicity to plants (phytotoxicity) may vary, and so ranges are usually reported. Table 10-1
SALTS AND TRACE ELEMENTS 382 FIGURE 10-5 Total selenium concentrations in the top 30.5 cm (12 inches) of soil (A) and in shallow groundwater (B) from 1984 to 1989 in the San Joaquin Valley. Source: San Joaquin Valley Drainage Program. 1990. A Management Plan for Agricultural Subsurface Drainage and Related Problems on the Westside San Joaquin Valley. Final Report. Sacramento, Calif.: San Joaquin Valley Drainage Program.
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SALTS AND TRACE ELEMENTS 384 FIGURE 10-6 Heavy metal contents in Greenfield sandy loam treated with composted sludge from 1976 to 1981. Source: A. C. Chang, J. A. Warneke, A. L. Page, and L. J. Lund. 1984. Accumulation of heavy metals in sewage sludge-treated soils. Journal of Environmental Quality 13:87â91. Reprinted with permission from © American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America.
SALTS AND TRACE ELEMENTS 385 TABLE 10-1 Concentration of Trace Elements Commonly Observed in Forage Crops Dry Weight (mg/kg) Element Typical Phytotoxic Arsenic 0.01â1.0 3â10 Boron 7â75 75 Cadmium 0.10â1.0 5â700 Cobalt 0.01â0.3 25â100 Copper 3â20 25â40 Molybdenum 0.10â3.0 100 Nickel 0.10â5.0 50â100 Selenium 0.10â2.0 100 Zinc 15â150 500â1,000 SOURCE: A. L. Page, A. C. Chang, and D. C. Adriano. 1990. Deficiencies and toxicities of trace elements. Pp. 138â160 in Agricultural Salinity Assessment and Management, K. K. Tanji, ed. ASCE Manuals and Reports on Engineering Practice No. 71. New York: American Society of Civil Engineers. Reprinted with permission from © American Society of Civil Engineers. summarizes the concentration ranges of trace elements associated with both normal and phytotoxic levels in forage crops (Page et al., 1990). Table 10-2 presents the recommended maximum concentrations of 15 trace elements in irrigation waters for long-term protection of plants and animals (Pratt and Suarez, 1990). These concentrations should be considered as guidelines designed to protect the most sensitive crops and animals from receiving toxic amounts of trace elements. Figure 10-7 gives the range of the selenium concentrations found in edible portions of crops and forage plants grown in selenium-impacted soils of the San Joaquin Valley's west side (Tanji, 1991b). These levels are not high enough to contribute amounts above normal dietary levels (50 to 200 µg/day [0.002 to 0.0007 ounces] for adults). However, some types of crops like the crucifers (for example, cabbage and mustard) are capable of assimilating high levels of selenium; this may be of concern in the future because agricultural and drainage practices will probably change in the west side of the San Joaquin Valley. Furthermore, public health officials give health advisories to rural residents who live where selenium levels are high. Those residents are advised to limit their consumption of home-grown foodstuffs. Chang and colleagues (1984) applied soil sludge to winter barley and sorghum crops for 6 years (Figure 10-6). Table 10-3 summarizes the total amounts of cadmium and zinc removed by crops treated with the two
SALTS AND TRACE ELEMENTS 386 TABLE 10-2 Recommended Maximum Concentrations of 15 Trace Elements in Irrigation Waters for Long-Term Protection of Plants and Animals
SALTS AND TRACE ELEMENTS 387 Element Recommended Maximum Comments Concentration (mg/liter)a Molydenum 0.01 This concentration is below the phytotoxic level but is recommended to protect animals from molybdosis because of excess molybdenum in forages. Nickel 0.20 Nickel is toxic to many plants at concentrations of 0.5 to 1.0 mg/ liter. Toxicity from this element decreases with an increase in pH, so acidic soils are the most sensitive. Selenium 0.02 This guideline protects livestock from selenosis because of selenium in forage. Selenium absorption by plants is greatly inhibited by sulfate, so the guideline for this element can be increased for gypsiferous soils and waters. Vanadium 0.10 Toxicity to some plants has been recorded at vanadium concentrations above 0.5 mg/liter. Zinc 0.50 Zinc is toxic to a number of plants at a concentration of 1 mg/liter in nutrient solution, but soils have a large capacity to precipitate this element. This guideline is designed to provide protection for acidic sandy soils. Neutral and alkaline soils can accept much greater concentrations without developing toxicities. types of sludge applications. Winter barley and sorghum removed increasing amounts of cadmium and zinc with increasing sludge applications, with zinc uptake being far greater than cadmium uptake. The amount of metals taken up by the crops was insignificant (less than 1 percent of that applied). However, the metal contents in the crops were at phytotoxic levels (Table 10-1) and probably should not be consumed by animals including humans. a Loading rates in kg/ha-year can be calculated from the relationship that 1 mg/liter in the water gives 10 kg/ha-year when water is used at a rate of 10,000 m3/ha-year. b For citrus, the maximum recommended concentration is 0.075 mg/liter. SOURCE: P. F. Pratt and D. L. Suarez. 1990. Irrigation water quality assessments. Pp. 220â236 in Agricultural Salinity Assessment and Management, K. K. Tanji, ed. ASCE Manuals and Reports on Engineering Practice No. 71. New York: American Society of Civil Engineers. Reprinted with permission from © American Society of Civil Engineers. ALTERNATIVE MANAGEMENT OPTIONS Although water quality problems induced by irrigation tend to share some common features (National Research Council, 1989b), problems caused by site- specific conditions and processes prevail. Soils and
SALTS AND TRACE ELEMENTS 388 cropping systems, irrigation and drainage systems, water rights, institutional infrastructure, and drainage water disposal practices differ. Nevertheless, it is possible to generically group alternative management options that can be used to control salinity and trace elements into (1) source control measures, (2) drainage water reuse, (3) drainage water treatment, (4) drainage water disposal, and (5) institutional changes. FIGURE 10-7 Concentrations of selenium in tissues of various edible crops. Source: K. K. Tanji. 1991. Principal Accomplishments 1985â90. Davis: University of California, Division of Agriculture and Natural Resources, UC Salinity/ Drainage Task Force.
SALTS AND TRACE ELEMENTS 389 TABLE 10-3 Total Removal by Crops of Cadmium and Zinc from Sludge-Treated Greenfield Sandy Loam Soils, 1976-1981a Cumulative Solids Applied in Sludge (metric ton/ha) Concentration in Soil (g/ha) Cadmium Zinc Composted sludge 0 5.6 1,199 137 15.0 2,166 274 35.0 3,049 548 55.7 3,877 Liquid sludge 0 7.1 1,170 80 84.5 3,534 156 106.9 4,650 298 129.4 5,765 a The total amounts of cadmium and zinc were calculated as the sum of annual crop yields (grain and straw) multiplied by their corresponding metal contents. SOURCE: A. C. Chang, J. A. Warneke, A. L. Page, and L. J. Lund. 1984. Accumulation of heavy metals in sewage sludge-treated soils. Journal of Environmental Quality 13:87â91. Reprinted with permission from © American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America. Source Control Measures The principal aim of source control is to use water and land resources efficiently with off-farm and on-farm measures that minimize salinity and trace element problems. The off-farm or irrigation project measures involve flexibility, reliability, and stream flow control in water delivery to the farm (Clemmons, 1987). A rotation schedule may range from fixed to seasonally varied water deliveries with regard to amounts and timing. In contrast, demand and arranged schedules allow farmers to have complete flexibility in the frequency, rate, and duration of water delivery. The use of modern technology that uses automated downstream or upstream canal regulation and centralized computer controls is desirable but costly. Substantial improvements in the timing, flow, and volume of water deliveries to farms may be achieved by well- trained ditch riders (personnel responsible for monitoring and adjusting water control gates in irrigation canals). Seepage of water from canals is a major problem, particularly in salt-affected areas, where seepage water picks up dissolved mineral salts from saline soils and geologic formations, for example, in the Grand Valley of Colorado.
SALTS AND TRACE ELEMENTS 390 On-Farm Source Control Measures The on-farm source control measures for salinity and trace elements involve the application of water more uniformly and efficiently to reduce surface as well as subsurface drainage. Irrigation scheduling to determine the timing and amount of irrigation water application is essential. Scheduling may be achieved by one of two general methods: (1) by monitoring soil and/or crop parameters and (2) by computing the soil water balance. The first method involves measuring the soil water content or matrix potential (a measure of how tightly water molecules are bound to soil particles) and/or the leaf water potential of the crop. The second method requires an estimate of the storage capacity of water in soil, crop rooting depth, allowable soil water depletion, and crop evapotranspiration (Martin et al., 1990). Such water balance techniques range from simple ''checkbook" accounting to computerized scheduling using real-time weather data like those provided by the California Irrigation Management Information System or the model of the Agricultural Research Service, U.S. Department of Agriculture. Water Application Systems Water is applied to croplands by surface and pressurized irrigation application systems. The surface irrigation systems include furrow, border, and basin methods. The pressurized systems include highand low-volume sprinklers and surface and subsurface drip or trickle irrigation systems. Each of these water application methods has its advantages and disadvantages, depending on site-specific conditions and agronomic practices. The performance characteristics of irrigation systems can be evaluated by measuring their application efficiency uniformities, deep percolation ratios, and tailwater ratios (Heerman et al., 1990). The potential attainable application efficiencies for these irrigation systems are nearly the same as those for properly designed and managed systems. However, the typical application efficiencies of surface methods tend to be lower (50 to 70 percent) than those of sprinklers and drip/ trickle systems (75 to 90 percent). Improved furrow irrigation systems with shorter runs (for example, 200 m [218 yards]), modified set times, and tailwater return systems can achieve the application efficiencies of pressurized systems. Management of Salt-Affected, Waterlogged Croplands Salt-affected, waterlogged croplands require additional considerations and special management practices. Reductions in subsurface drainage
SALTS AND TRACE ELEMENTS 391 minimize off-farm environmental impacts. In addition to the distribution uniformity of water application, subsurface drainage is affected by the uniformity of water infiltration rates across irrigated fields. Deep-rooted, salt- tolerant crops may be able to use shallow groundwaters to meet a portion of their evapotranspiration needs. The salt balance in the crop root zone needs to be maintained to sustain crop production. When the source of soil salinity is the salts contained in the irrigation water, the guidelines of Ayers and Westcot (1985) can be used to establish a certain leaching fraction for salt control. If, however, the principal source of salts is naturally occurring salts in the soils, soil salinity needs to be monitored and water in excess of crop water needs to be applied periodically to control salinity in the root zone. Because of the nonuniformity of both application of water and infiltration rates, subsurface drainage water and associated pollutants will be produced even if producers implement best-management practices. Moreover, salt accumulation in the root zone from either applied water or chemical weathering of soils needs to be controlled to sustain crop yields. Thus, irrigated agriculture inevitably results in the production of residuals (drainage water and pollutants) that need to be managed and/or disposed. Drainage Water Reuse Source control can be viewed as the first line of action to reduce the off- site impacts of return flows of water from irrigation. A second management option is to reuse the subsurface drainage water until it is no longer usable. The San Joaquin Valley Drainage Program (1990) recommended a drainage water reuse strategy in waterlogged lands. High-quality irrigation water is used to irrigate the more salt-sensitive crops. The subsurface drainage from salt- sensitive crops is used to irrigate the more salt-tolerant crops and trees. The subsurface drainage from salt-tolerant plants is, in turn, reused to irrigate halophytes (plants that tolerate elevated salinities). In this strategy, the volume of drainage water is successively decreased, and concurrently, the salinity of the drainage water is successively increased so that further management of subsurface drainage waters could be carried out more efficiently, for example, disposal in off-site water bodies, salt harvesting in evaporation ponds, water treatment, or injection into deep-wells. Saline water as either fresh irrigation water or subsurface drainage water has been successfully used under certain conditions. For instance, irrigation water containing an average of 2,500 mg/L of total dissolved solids (2,500 ppm) has been used for decades in the Pecos Valley of
SALTS AND TRACE ELEMENTS 392 Texas (Moore and Hefner, 1977). In contrast, the Broadview Water District in the San Joaquin Valley's west side blended subsurface drainage water into fresh canal water for about 25 years, but it had to discontinue this practice. The blended water initially had an EC of 1.6 dS/m in 1956, but the EC rose to 3.2 dS/m by 1981, with a decline in the growth and yields of the more salt-sensitive crops. A major problem was the crusting of surface soils as a result of using waters with high sodium adsorption ratios (see above) and difficulties in seed germination and seedling growth. Agroforestry is a new approach being tested in the San Joaquin Valley to lower high water tables through water extraction by tree roots as well as the reuse of saline drainage waters. Tanji and Karajeh (1991) have intensively monitored salt and water balances in a 9.4-ha (23.2-acre) eucalyptus plantation. The 6-year-old trees lowered the water table from about 0.6 to 2.2 m (2 to 7 feet) below the soil surface and are using irrigation drainage water with an EC of 10 dS/m from nearby croplands. However, a buildup of salinity to an average ECe of 25 dS/m has reduced the evapotranspiration rate by about 67 percent. The 16 percent leaching fraction has been increased to reduce soil salinity and improve the evapotranspiration rates of trees. Drainage Water Treatment The California Department of Water Resources has investigated desalinization of agricultural drainage waters in the San Joaquin Valley. Reverse osmosis shows the greatest potential for achieving this (Lee, 1990). To successfully desalt drainage waters, however, the drainage waters must be pretreated to avoid scaling and biological fouling of the membranes used in the reverse osmosis process. Pretreatment involves removal of suspended solids, silica, and calcium and sulfate ions. Disinfection is also required. The estimated cost for reverse osmosis is $880/103 m3 ($1,090/acre-foot), excluding the cost of collecting and delivering the drainage water and disposing the treatment by- products (Lee, 1990). Biological and Physicochemical Processes The San Joaquin Valley Drainage Program sponsored research on the removal of selenium and other trace elements from agricultural drainage waters, including biological and physicochemical processes. An anaerobic bacterial process used methanol as a source of carbon for microbes to reduce selenium, microfilters to remove fine suspended solids, and
SALTS AND TRACE ELEMENTS 393 ion-exchange resins to polish the effluent. The 330 to 550 µg/liter of selenium of influent (330 to 550 ppb) was reduced to 16 to 50 µg/liter (16 to 50 ppb) in the biological reactor and to 10 to 40 µg/liter after microfiltration. Treatment costs ranged from $118 to $182/103 m3 ($145 to $224/acre-foot) (Lee, 1990). A second biological process that has been studied involved growth and harvesting of microalgae and bacteria, methane fermentation of the biomass, and ferric chloride treatment. In laboratory studies, the digested biomass reduced the selenium concentration in the influent from 367 to 20 µg/liter (367 to 20 ppb). In field studies, incorporation of a nitrate reduction process and treatment with ferric chloride further reduced the selenium level to less than 1 µg/liter (1 ppb). The treatment cost for this microalgal-bacterial process ranged from a conservative $55 to $83/103 m3 ($67 to $102/acre-foot) (Lee, 1990). A third biological process involves the volatilization of methylated selenides by several indigenous species of fungi. This volatilization process is applicable to both ponded waters and surface soils. Further research is needed to assess the volatilization of selenium from ponded waters (Lee, 1990). The physicochemical treatment processes that have been investigated include chemical reduction and surface adsorption of selenium onto hydroxylated surfaces. Selenium can be reduced and precipitated from drainage waters by using heavy doses of ferrous hydroxide. The treatment costs for reducing selenium concentrations to 1 µg/liter (1 ppb) range from $57 to $125/103 m3 ($70 to $154/acre-foot) (Lee, 1990). Selenium can also be adsorbed to iron filings activated by oxygenation. Apparently, both surface adsorption to hydroxylated sites as well as chemical reactions aid in immobilizing selenium up to about 90 percent of the initial concentration. Under field conditions, serious problems of cementation occurred in the bed of iron filings. Depending on the life expectancy of the bed, the costs for reducing selenium concentrations ranged from $57 to $231/103 m3 ($70 to $284/acre-foot) (Lee, 1990). Future Research Needs The treatment studies described above were carried out in bench-scale models in the laboratory and in mini-pilot plants in the field. Although many of these processes show some promise in their ability to remove selenium, there remains a need for further research to understand the basic mechanisms by which selenium can be removed (Lee, 1990). The costs of these treatments are likely too expensive for irrigators to bear
SALTS AND TRACE ELEMENTS 394 the entire costs. The anaerobic bacterial treatment process appears to show the greatest promise of removing selenium at a practical cost, and a pilot plant study is under way in the San Joaquin Valley (Lee, 1990). Drainage Water Disposal Options for disposing of agricultural drainage waters include (1) deep percolation into the underlying groundwater basin; (2) discharge into surface waters, with the ultimate destination being the oceans or inland sinks; (3) disposal in agricultural evaporation ponds; and (4) deep-well injection into permeable substrata. The first option was discussed earlier in this chapter. Discharge into Surface Waters The practice of discharging collected surface irrigation return flow water into streams and lakes is widespread. However, increasing constraints are being placed on such discharges as more stringent water quality standards for receiving waters are being promulgated. Thus far, irrigation drainage water is considered a nonpoint source of pollution and is not regulated as much as point sources of discharge are. Increasing constraints will likely be placed on nonpoint sources of pollution. Disposal in Agricultural Evaporation Ponds An example of disposal into agricultural evaporation ponds is the Kesterson Reservoir. Such a practice may pose risks to wildlife and groundwater. The 510-ha (1,260-acre) Kesterson Reservoir was constructed primarily to evaporate impounded saline drainage water and secondarily to maintain a habitat for waterfowl. (The impact of selenium accumulation in the aquatic food chain was discussed above.) A study done before reservoir construction indicated that about 40 percent of the impounded water would seep into the underlying aquifer (Benson et al., 1990). While Kesterson was under operation, a groundwater mound formed about 0.5 to 3.0 m (1.6 to 9.8 feet) above the regional groundwater level. The rate of lateral groundwater flow was about 4.6 m/year (15 feet/year). Much of the selenium present in the impounded water accumulated in the sediments and organic detritus. The selenium concentrations in shallow groundwater were low because of the transformation of oxidized forms of selenium to reduced forms (elemental selenium and selenides), which are immobile.
SALTS AND TRACE ELEMENTS 395 Elsewhere in the San Joaquin Valley's west side, agricultural evaporation ponds were installed between 1972 and 1985, mainly in the environs of Tulare Lake Basin, which is a hydrologically closed basin. Of the 28 ponds constructed, 5 are now inactive or closed. These ponds occupy a surface area of about 2,800 ha (about 7,000 acres) and vary from 4 to 720 ha (10 to 1,800 acres) (Tanji and Dahlgren, 1990). The evaporation ponds annually receive about 3,900 ha-m (31,600 acre-feet) of subsurface drainage from some 22,400 ha (55,350 acres) of tile-drained lands. The drainage waters discharged into the ponds annually contain about 0.72 million metric tons (0.8 million tons) of dissolved mineral salts, which is equivalent to about 25 percent of the annual salt accumulation in the west side of the San Joaquin Valley. The selenium concentrations in the drainage waters disposed into these ponds vary from less than 1 to 610 µg/liter (1 to 610 ppb). The average concentration of selenium disposed into the Kesterson Reservoir was about 300 µg/liter (300 ppb). The evaporation pond facility receiving 610 µg/liter of selenium (610 ppb) was shut down in 1989 because the selenium concentrations in the evapoconcentrating water exceeded 2,000 µg/liter (2,000 ppb), and the water was judged to be hazardous liquid waste. Selenium poisoning symptoms like those in the Kesterson Reservoir have been detected in several agricultural evaporation ponds that receive influent selenium concentrations at much lower levels (10 to 80 µg/liter [10 to 80 ppb]). The differences in toxicity threshold levels found between the Kesterson Reservoir and the agricultural evaporation ponds are attributed to site-specific conditions. For instance, the Kesterson Reservoir is located in an area surrounded by uncontaminated wetlands where some feeding by waterfowl took place. In contrast, the Tulare Lake Basin ponds are the only surface water bodies present and are the principal habitats and feeding grounds for waterfowl. Because of these new findings on the hazardous nature of evaporation ponds (Skorupa and Ohlendorf, 1991), this practice of drainage water disposal is expected to be severely curtailed for drainage waters containing potentially toxic amounts of trace elements. Deep-Well Injection Deep-well injection, similar to the disposal of waste brine from oil fields, is another option for disposing agricultural drainage waters. A 2,400-m (7,900- feet)-deep well was constructed in the Westland Water District of the San Joaquin Valley to test this option. After drilling, injection tests indicated that the subsurface geologic formation had a
SALTS AND TRACE ELEMENTS 396 substantially lower than expected permeability. A shallower formation with higher permeability is present, but the EPA has not approved this proposed deep-well injection. The service life of an injection well is governed largely by the buildup of water pressure in the aquifer as well as the physical and chemical properties of the aquifer (Lee, 1990). Particulate matter, chemical precipitation, and biological slime at the injection site can cause clogging. Thus, pretreatment appears to be a necessary process for deep-well injection of agricultural drainage waters. The estimated cost for this option is $132 to $172/103 m3 ($164 to $213/acre-foot). Institutional Changes Institutional changes that can be used to solve drainage water quality problems include (1) land retirement or idling, (2) tiered water pricing, (3) water marketing and transfers, and (4) the use of regional drainage management authorities. Land Retirement or Idling One suggestion made by the San Joaquin Valley Drainage Program (1990) is to cease irrigation of areas where shallow groundwaters contain elevated levels of selenium and where it is difficult to drain the soils. Such lands could be permanently retired or idled until some future date. Conceptually, this option may be an attractive relative to other alternatives of source control, treatment, or disposal. However, specific criteria for land retirement have not yet been developed, and only preliminary economic analyses have been carried out. Tiered Water Pricing Tiered water pricing or increasing block-rate prices for irrigation water may serve as a motivation to reduce the amount of drainage water. For instance, the Broadview Water District in the San Joaquin Valley implemented such a program in 1989 on a trial basis. Prior to this new rate structure, water was sold at $13/103 m3 ($16/acre-foot). Crop-specific average water application rates were established, and any additional use above these basic rates were charged at $32/10 3 m3 ($40/acre-foot). The district offered real-time weather data to growers so that they could better schedule irrigation of their crops. The estimated reduction in drainage water was about 23 percent (Wichelns, 1991). Part of this reduction may be attributed to a drought-related decrease in the water supply.
SALTS AND TRACE ELEMENTS 397 Water Marketing and Transfer Water marketing and transfer may provide, in some instances, an economic incentive to irrigators to consider off-farm uses of water. It requires a clear arrangement between the buyer and seller of the rights to transfer water for a limited period or longer. Such voluntary transfer of water, however, may create third-party impacts that have not been studied adequately. In California's fifth year of drought (1991), the California Department of Water Resources actively solicited water and was successful in arranging about 49,000 ha-m (400,000 acre-feet) of one-time water transfers to drought-stricken urban and agricultural water users. Water marketing for environmental benefits has been somewhat limited. For instance, 3,700 ha-m (30,000 acre-feet) of fresh water was purchased in 1989 by the California Department of Fish and Game and the Grasslands Water District from the U.S. Bureau of Reclamation to maintain wildlife and fish (Robert Potter, deputy director, California Department of Water Resources, personal communication, 1991). Existing state and federal laws relative to water rights may need to be reassessed to promote water marketing and transfers. Regional Drainage Management Authorities Formation of regional drainage management authorities may aid in regulating drainage water production through, for instance, penalty costs on drainage water or subsidies for drainage water reductions. It is not unusual, however, to have an entity that delivers irrigation water and another entity that manages drainage water in a given region but with different boundaries. There is a need for joint planning and management of irrigation and drainage waters. Some efforts are being made to form drainage management authorities in the San Joaquin Valley.
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